1. Field of the Invention
The present invention relates to an electroluminescence device and a process for producing the same, and for example, an electroluminescence device, which is a self-emission type flat display, which is particularly suitable for a display device or an emission device, such as an organic electroluminescence color display using an organic thin film as an electroluminescence layer, and a process for producing the same.
2. Description of the Related Art
The importance of an interface between a human being and a machine is recently being increased, such as in multimedia oriented commercial products. In order that a human being comfortably operates a machine with high efficiency, a sufficient amount of information must be withdrawn from the machine to be operated simply in a moment, and accordingly investigations of various display devices such as a display are being conducted.
It is the current situation in that a demand of miniaturization and thinning of a display device are being increased day by day along with miniaturization of the machine.
For example, there is notable progress in miniaturization of a lap-top type information processing apparatus, to which a display device is integrated, such as a note type personal computer and a note type word processor, and accordingly technical innovation in a liquid crystal display for its display device is also considerable.
A liquid crystal display is currently used as an interface of various products, and frequently used in products that are routinely used, such as a compact television set, a watch and a portable calculator, in addition to the lap-top type information processing apparatus.
Owing to the characteristic feature of a liquid crystal in that it can be driven with a low voltage and consumes low electric power, the liquid crystal display has been studied as the center of an interface between a human being and a machine for compact to large scaled display devices.
However, since the liquid crystal display is not of self-emission type, it requires a backlight, and a larger electric power is required to drive the backlight than to drive the liquid crystal. As a result, an operation time becomes short with a self-contained battery, which causes a limitation on use.
Furthermore, it is also a problem in that since the viewing angle of the liquid crystal display is narrow, it is not suitable as a large scaled display device, such as a large display.
It is also considered as a big problem in that since the display method of the liquid crystal display depends on the orientation state of liquid crystal molecules, the contrast changes depending on the angle even within the viewing angle.
From the standpoint of the driving method, an active matrix method, one of the driving methods, exhibits a response time sufficiently high as handling an animated image, but because it uses a TFT (thin film transistor) driving circuit, a large scaled display size is difficult to be obtained due to pixel fault.
In the liquid crystal display, a simple matrix method, another driving method, is low in cost and can rather easily produce a large display size, but it does not have a response time sufficiently high as handling an animated image.
On the other hand, as a self-emission type display device, a plasma display device, an inorganic electroluminescence device and an organic electroluminescence device are studied.
A plasma display device uses plasma emission in a low pressure gas for display, and is suitable for a large-scale or large capacity display, but involves a problem in thinning and cost. It also requires an alternative current bias of high voltage, and is not suitable for a portable apparatus.
While an inorganic electroluminescence device has been shipped as a commercial product, such as a green emission display, it is driven by alternative current bias as similar to the plasma display device, which requires a voltage of about several hundreds volt, and lacks practical utility.
However, with development in technology, it has been succeeded to emit the three primary colors, R (red), G (green) and B (blue), necessary for displaying a color image, but since it comprises an inorganic material, control of the emission wavelength by molecular designing is difficult, and it is considered that production of a full color display is difficult.
On the other hand, an electroluminescence phenomenon by an organic compound has been studied for a long time since the discovery of the emission phenomenon by carrier injection to an anthracene single crystal that generates strong fluorescence in the first half of 1960s. However, since it is of low luminance with a single color and requires a single crystal, it has been conducted as fundamental studies on carrier injection to an organic material.
However, since 1987 when Tang et al of Eastman Kodak reported an organic thin film electroluminescence device having a laminated structure having an amorphous emission layer that could be driven with a low voltage and could emit with high luminance, its studies and developments have been conducted in various field for emission of three primary colors, R, G and B, stability, increase in luminance, laminated structures, production processes, etc.
Various novel materials have been developed by a design of molecule, etc., which is a characteristic feature of an organic material, and application of an organic electroluminescence display device having superior characteristics, such as driving with low voltage direct current, a thin form and self-emission, is being studied for a color display.
An organic electroluminescence device (hereinafter sometimes called as an organic EL device) has a film thickness of 1 xcexcm or less, and has ideal characteristics for a self-emission type display device, such as emission from an area with converting electric energy to light energy by injection of an electric current.
FIG. 1 shows an example of a conventional organic EL device 10. The organic EL device 10 is produced by film formation of an ITO (indium tin oxide) transparent electrode 5, a hole transfer layer 4, an emission layer 3, an electron transfer layer 2, and a cathode (for example, an aluminum electrode) 1 on a transparent substrate (for example, a glass substrate) 6 in this order by a vacuum evaporation method.
By applying an direct current voltage 7 between the transparent electrode 5, an anode, and the cathode 1, a hole as a carrier injected from the transparent electrode 5 migrates through the hole transfer layer 4, and an electron injected from the cathode 1 migrates through the electron transfer layer 2, and as a result, electron-hole recombination occurs to form emission 8 of a prescribed wavelength, which can be observed from the side of the transparent substrate 6.
An emission substance, such as anthracene, naphthalene, phenanthrene, pyrene, chrysene, perylene, butadiene, coumarin, acridine, stylbene, etc., may be used as the emission layer 3. This may be contained in the electron transfer layer 2.
FIG. 2 shows another conventional example, which is an organic EL device 20, in which the emission layer 3 is omitted, and the emission substance is contained in the electron transfer layer 2 as described above, so that the emission 18 of a prescribed wavelength is formed at the boundary between the electron transfer layer 2 and the hole transfer layer 4.
FIG. 3 shows a specific example of the above-described organic EL device. It is constituted in such a manner that a laminated body of the organic layers (the hole transfer layer 4, the emission layer 3 and the electron transfer layer 2) is provided between the cathode 1 and the anode 5; these electrodes are formed in the form of stripes that cross each other to form a matrix; a signal voltage is applied in time sequence by a brightness signal circuit 34 and a controller circuit 35 having a shift register, to form emission at each of the numerous points of intersection (pixels).
Therefore, by using such a constitution, it can be used as an image reproducing apparatus, in addition to a display. It can be constituted as for a full color or multi-color display by arranging the pattern of the stripes for each of the colors, R, G and B.
In the display device comprising plural pixels using such an organic EL device, the organic thin film layers 2, 3 and 4 are sandwiched by the transparent electrode 5 and the metallic electrode 1, and emit light to the side of the transparent electrode 5.
However, the organic EL device still has problems unsolved.
Stable emission of the three primary colors, R, G and B is the necessary condition of the application of the organic EL device to a color display. When different series of material are used in these three systems of R, G and B in the process for producing the device, the production steps become extremely complicated and a long period of time is required.
An object of the invention is to provide an electroluminescence device that can be easily produced with a low cost and has a device structure capable of conducting stable emission, and a process for producing the same.
The inventors have conducted earnest investigations for the above-described circumstances, and have found that the device can be easily produced with a low cost by using the common material as possible in at least three kinds of laminated bodies having emission regions for the respective colors, so as to attain the invention.
Accordingly, a first aspect of the invention relates to an electroluminescence device comprising at least three kinds of laminated bodies, in each of which an emission region is independently present in each of a hole transfer layer and an electron transfer layer, the laminated bodies comprising the hole transfer layers each comprising a layer comprising a common material, and the electron transfer layers each comprising a layer comprising a common material, and the electroluminescence device emitting at least three colors of light.
A second aspect of the invention relates to an electroluminescence device comprising two kinds of laminated bodies, in each of which an emission region is independently present in each of a hole transfer layer and an electron transfer layer, the laminated bodies comprising the hole transfer layers each comprising a layer comprising a common material, and the electron transfer layers each comprising a layer comprising a common material, and the electroluminescence device emitting two colors of light.
According to the electroluminescence device of the invention, because the hole transfer layers and the electron transfer layers each comprise the common material layer within each of the laminated bodies, in which the emission region is independently present in each of the hole transfer layer and the electron transfer layer, the laminated bodies for each of the colors of emission light can be easily produced with a low cost. By forming the common layers on the whole surface of an effective pixel region by using a mask with a large opening, the film formation property and the step coverage property become good, and a leakage current between the cathode and the anode can be reduced.
The invention also provides a process for producing an electroluminescence device, as a process for producing the electroluminescence device of the first aspect of the invention with good reproducibility, which comprises
a step of forming a first electrode, which is common in at least three kinds of the laminated bodies, on a common substrate,
a step of forming the hole transfer layers by film formation of a common hole transfer layer forming material on a region containing at least three kinds of the laminated bodies on the first electrode,
a step of forming the electron transfer layers by film formation of a common electron transfer layer forming material on a region containing at least three kinds of the laminated bodies on a region containing the hole transfer layers, and
a step of forming second electrodes for at least three kinds of the laminated bodies on the electron transfer layers, to face the first electrode.
The invention also provides a process for producing an electroluminescence device, as a process for producing the electroluminescence device of the second aspect of the invention with good reproducibility, which comprises
a step of forming a first electrode, which is common in two kinds of the laminated bodies, on a common substrate,
a step of forming the hole transfer layers by film formation of a common hole transfer layer forming material on a region containing two kinds of the laminated bodies on the first electrode,
a step of forming the electron transfer layers by film formation of a common electron transfer layer forming material on a region containing two kinds of the laminated bodies on a region containing the hole transfer layers, and
a step of forming second electrodes for two kinds of the laminated bodies on the electron transfer layers, to face the first electrode.